Introduce yourself: Hi, this is [ton nom].
The reason why you are calling: I am calling because I cannot come into work today, since my car broke down.
Your polite request: I was wondering if I could pick up a shift tomorrow instead, which would leave me some time to fix my car.
Your phone number: Please call me back at [ton numéro de téléphone] to confirm if that is ok with you.
End your conversation: Thank you so much and have a great rest of your day!